Heat pump technology is well accepted to be more energy efficient by transferring heat from air or water to air or water using common refrigeration technology. Water-based heat transfer is inherently more efficient that air-based transfer due to the higher density of water with more mass to carry heat. Water-to-water heat pumps are the most efficient of all heat pump designs, thus the growing popularity of geothermal heating and cooling systems which move heat to or from the mass of the earth versus the more common air-to-air outdoor compressor. The energy savings can be as high as 70% for home heating or cooling. Heat pump water heaters provide energy and a cost efficient way to heat water with electricity. These types of heaters typically provide the same amount of hot water as electric resistance heaters, but do so at one-half to one-third the energy cost. Heat pump water heaters may also have the added benefit of providing air-conditioning as a by-product of water heating.
Heat pump water heaters work by transferring heat, not by generating heat. Typically, a heat pump water heaters uses a standard vapor refrigeration compression cycle in reverse, In this manner, a heat pump water heater uses a closed-loop heat exchange circuit to absorb heat from a source (such as air in a room) and transfers the heat to a heat sink (such as water in a water storage tank). The energy consumed in a heat pump water heater system is the energy to run a compressor to circulate the refrigerant in the heat exchange circuit.
U.S. Pat. No. 7,334,419 describes such a heat pump water heater which generally relates to a water heater that heats water in a storage tank and heating or cooling ambient air. However, similar heat pump water heaters often require expensive installation procedures and do not address the shortcomings of reducing overall heating costs for domestic water uses.
Other heating and cooling strategies include devices which rely on low pressure geothermal steam and a turbo generator to which steam is applied. U.S. Pat. No. 6,212,890 describes a geothermal power plan and condenser which utilizes such a system in producing energy. The low pressure geothermal steam is applied to the turbine, wherein expansion takes place driving the generator, and producing expanded steam that is exhausted from the turbine. A condenser for condensing the expanded steam includes a steam heat exchanger for receiving the expanded steam and a fan for cooling the expanded steam. One shortfall of such devices however, is the need for a constant source of geothermal steam as a power source as well as the costs associated with maintaining steam driven turbines and condensers.
Similarly, U.S. Pat. No. 4,102,133 describes a multiple well dual fluid geothermal power generator which purportedly transfers heat by exchange into a power fluid cycle, at different points in the cycle. The end result being an increase in the amount of power which can be developed per unit of geothermal fluid supplied. However, as with the '890 patent, this device requires a constant source of geothermal steam as well as the maintenance costs associated with such a device.
U.S. Pat. No. 7,234,314 describes a geothermal heating and cooling system with solar heating, which purportedly improves the efficiency and installation costs functionality of predecessor geothermal heat exchange designs for closed-loop water source heat pump systems, and for direct expansion heat pump systems. As described, efficiency is improved by means of a solar heat collector solar system that is operatively connected to a new, or to an existing, geothermal direct expansion, or closed-loop water-source, heat pump system so as to provide supplemental heat in the heating mode, when advantageous, by means of natural solar heat collector fluid convection. Such systems however, only provide a supplemental source of solar heating to new or existing geothermal direct expansion, or closed-loop water-source heat pump systems.
Likewise, U.S. Pat. No. 4,382,368 provides a geothermal hot water system which includes a hot water tank and a warm water tank which are heated independently of each other by a close loop Freon system. The closed loop Freon system includes a main condenser which heats water for the warm water tank and a super-heated condenser which heats water for the hot water tank. Generally, the hot water system extracts heat from a water main such as well water which is subsequently discharged with no environmental change and is carried to the water to be heated by condensing liquid such as Freon.
Typically, electric water heaters are very expensive to operate, requiring 4,000 kWh per year or $60.00 to $120.00 per month for a family in most areas. The present invention would cost about one-fourth of that which is the same as, or less than, a natural gas water heater costs to operate.
When it is available, natural gas is the most popular choice of fuel for water heating because it costs less than electricity. Natural gas is not available in all areas and requires a costly infrastructure element to add to a community. While currently less expensive to operate, natural gas water heaters unavoidably create carbon dioxide emissions. Electric water heaters can be powered with carbon-free fuel.
Ground source heat exchange systems typically utilize liquid-filled closed loops of tubing buried in the ground, or submerged in a body of water, so as to either absorb heath from, or to reject heat into, the naturally occurring geothermal mass and water surrounding the buried or submerged tubing. Contrary, water-source heat pump heating and cooling systems typically circulate water, or water with anti-freeze, in plastic underground geothermal tubing so as to transfer heat to or from the ground, with a second heat exchange step utilizing a refrigerant to transfer heat to or from the water, and with a third heat exchange step utilizing a compressor and an electric fan to transfer heat to or from the refrigerant to heat and cool interior space. Further, water-source heating and cooling systems typically utilize closed-loop or open-loop plastic tubing.
Closed-loop systems, often referred to as ground loop heat pumps, typically consist of a supply and return, ¾ inch to 2 inch diameter, plastic tube, joined at the extreme ends via an elbow, or similar, connection. The plastic tubing is typically of equal diameter, wall thickness, and composition, in both the supply and return lines. The water is circulated within the plastic tubing by means of a water pump. In the summer, interior space heat is collected by a common interior compressor and air heat exchanger system, or air handler, and is rejected and transferred into the water line via a refrigerant line to water line heat exchanger. In a similar manner in the winter, heat is extracted from the water line and transferred to the interior conditioned air space via the refrigerant liquid within the refrigerant line being circulated in a reverse direction. Many such systems are designed to operate with water temperature ranges of about a 10° F. water temperature differential between the water entering and exiting the heat exchange unit's copper refrigerant transport tubing. Water temperatures are often designed to operate in the 40 to 60° F. range in the summer and in the 25-45° F. range in the winter with anti-freeze added to the water.
In cases where a 1.5 inch diameter, plastic water conducting tubing is installed in a horizontal fashion about 5 or 6 feet deep, in 55° F. ground, about 200 to 300 linear feet per ton of system capacity may be necessarily excavated. If the same closed-loop plastic water conducting tubing is installed in a vertical borehole in 55° F. ground, about 150 to 200 feet per ton of system capacity may be drilled. In the horizontal style installation, the plastic tubing loop is typically backfilled with earth. In the vertical style installation, the plastic tubing loop inserted into the typical 5 to 6 inch diameter borehole is generally backfilled with thermally conductive grout. In either the horizontal or vertical style installation, a water pump is required to circulate the water through the tubing lines, which are generally of equal diameter in both the supply and return segments.
The basic operation of a heat pump water heater (HPWH) can be readily understood by examining it as a black box, without regard to the inner workings. Using this simplified approach, three energy flows, 1) heat removal (i.e. cooling), 2) electric energy and 3) water heating, are involved with a typical HPWH. The HPWH consumes electric energy and it removes heat from the heat source, producing a cooling effect. The energy gained is then delivered by the HPWH as heating output. For an air-source HPWH, the heat source is usually warm, humid interior air. Water-source heat pumps usually rely on a chilled water loop or a cooling tower loop as a heat source.
The electric energy input results in two useful effects: cooling and heating. The heating output (electrical input+heat removed from the heat source) is applied toward a water heating load. The cooling output is often used to cool and dehumidify the interior of a building. Since HPWHs have efficiencies greater than 100%, water heating efficiency for a HPWH is described by the coefficient of performance or COP, instead of using the term “efficiency.” The water heating COP is the ratio of the useful water heating output to the electric energy input.
Under typical conditions, an air-source HPWH delivers about 10,000 Btuh of water heating for every kilowatt of electric power it uses. It typically achieves a maximum temperature of about 130-150° F. depending on the refrigerant used. While heating water, the HPWH also provides a cooling effect of about 6700 Btuh per kilowatt. Like a conventional air conditioner, typically about 75% of the cooling output is sensible cooling and 25% is latent cooling or dehumidification at standard rating conditions.
The fundamental principles of operation for a HPWH are the same as those of a room air conditioner, a refrigerator, or an air-to-air heat pump. The basic functional components of a heat pump water heater are the evaporator, compressor, condenser, and expansion device.
Heat is transferred by the flow of refrigerant, taking advantage of the large amount of heat absorbed and released when the refrigerant evaporates and condenses. The flow of refrigerant is caused by the pressure differential created by the compressor. The compressor and condenser operate at higher pressure; that portion of the refrigeration system is called the high side. The portion containing the evaporator and the expansion device is called the low side. The compressor pulls refrigerant from the evaporator on the low side and discharges it to the condenser on the high side, much like a pump lifting water uphill. The expansion device resists the flow of refrigerant back to the low side, maintaining the pressure differential.
The refrigeration cycle is best understood by following a portion of the refrigerant around the cycle. The processes that occur in the major components are described in the following paragraphs.
Drawn by the compressor, refrigerant gas (vapor) leaves the evaporator at low pressure and low temperature and flows through the suction line to the compressor. As the compressor compresses the vapor to a higher pressure, its temperature rises (in the same manner as a bicycle pump becomes warm when pumping up a tire). Refrigerant leaves the compressor as a high-temperature gas at high pressure.
The compressor pushes hot, high-pressure refrigerant through the discharge line to the condenser. The condenser is simply a heat exchanger that removes heat from the hot gas and releases it to a heat sink (for HPWHs, the water being heated). The removal of heat from the hot gas causes it to condense to a liquid. Refrigerant leaves the condenser as an intermediate-temperature liquid at high pressure.
Liquid refrigerant flows from the condenser through the liquid line to the expansion device. By acting as a flow restrictor, the expansion device maintains high pressure on the condenser side and low pressure on the evaporator side. In larger commercial heat pump water heaters, the expansion device is an expansion valve. In smaller systems, it may be a capillary tube.
As the liquid moves through the expansion device, its pressure is suddenly lowered. The pressure drop causes some of the liquid refrigerant to flash (evaporate very quickly) into vapor. The evaporation of a portion of the liquid cools the remaining liquid, in the same way evaporation cools your skin when you step out of the shower. Refrigerant leaves the expansion device as a low-temperature mixture of gas and liquid at low pressure.
The cold, low-pressure mixture of liquid and gas refrigerant then flows to the evaporator. The evaporator is another heat exchanger that allows heat to move from a heat source (the air inside a building for most air-source HPWHs) to the refrigerant. As the liquid refrigerant evaporates to a gas, the evaporator removes heat from the heat source. The evaporator in an air-source HPWH provides a cooling and dehumidification effect for the building interior as the evaporator removes heat from the air. Dehumidification takes place only when the evaporator surface temperature is below the air's dewpoint temperature, allowing moisture to condense. Refrigerant leaves the evaporator as a low-temperature gas at low pressure, completing the cycle. The cycle is continuous while the machine is in operation, with refrigerant continuously moving through each part of the system.
A typical HPWH operating at normal conditions delivers about 10,000 Btuh of water heating for every kilowatt of electric power input (the equivalent of 3413 Btuh). About 15 gallons of water can be heated per hour through a temperature change of 80° F. The coefficient of performance for water heating is 10,000/3413 or about 2.9. In addition, about 6600 Btuh of cooling and dehumidification capacity is delivered for the same one-kilowatt input. This may be a desirable effect depending upon climate, season and location in house.
These rule-of-thumb values agree closely with the actual specifications for most specific HPWHs applied in typical conditions. As an example, one commercially available HPWH model uses five thousand Watts (17,100 Btuh) of electric power to deliver 50,000 Btuh of water heating and 31,500 Btuh of cooling. A total of 81,500 Btuh of useful heat flow is provided.
Coefficient of Performance (COP) is simply a measure of efficiency or the amount of useful output achieved for a given input. For example, an air-to-air heat pump might operate at a COP of three under favorable heating season conditions. This equates to a delivery of three units of heat to the building interior for each unit of energy consumed as electric energy.
HPWHs and other refrigeration devices are able to move more energy than they consume by taking advantage of the large amount of heat absorbed and released when the refrigerant evaporates and condenses. Air conditioners, refrigerators, freezers, and air-to-air heat pumps all operate similarly, with COPs normally about 1.7 to 3.2. However, they obtain a useful benefit only on one end of the heat transfer process
HPWHs use a small amount of electricity to upgrade the temperature of a large amount of heat and deliver it to meet a thermal load. The water heating efficiency of a heat pump water heater is always greater than 100%, and usually substantially greater. In addition to the water heating output, HPWHs often provide a cooling effect with no additional energy input.
At one-fourth the electric demand, the present invention allows less cost to power the geothermal water heater with photovoltaic panels as part of a grid connected PV system than to install a separate solar thermal system for water heating. Additional designs using a solar thermal heat source are also possible with the effect of significantly increasing solar performance by decreasing electric backup used and lowering the panel operating temperature.
The invention can also be installed in the place of most natural gas water heaters due to the lower electric service requirement for the heat pump which can operate on a standard 15 A-120V circuit. Otherwise, changing to electric water heating would require the installation of at least a 20 A-240V circuit, if available.